Gas generators, particularly those having application as emergency power units for military aircraft, are small, lightweight, intermittent duty power supply devices which ensure instant, reliable power output even after relatively long dormant periods. Such gas generators typically utilize a spray of liquid hydrazine into a catalyst bed such as Shell 405, in which production of hot gas occurs. The gas generated by the device may then be used to drive a turbine wheel, which may power such devices as engine starters, electrical generators, and hydraulic pumps.
Gas generators for military applications are designed to be reusable, with only a minimal amount of maintenance being required between uses. It is therefore imperative that the gas generator be capable of a consistent and reliable level of performance involving repeated usage over an extended period of time without requiring major overall, rebuilding, or replacement of the catalyst.
Liquid hydrazine is supplied to the gas generator at a pressure of approximately 100 p.s.i., and gas leaving the generator is at approximately the same pressure. The gas generator is therefore controlled by the amount of liquid hydrazine sprayed into the catalyst bed, which amount determines the volume of gas produced by the gas generator, and therefore controls the speed at which a turbine driven by the gas generator will operate.
In order to precisely control the operating speed of the turbine, liquid hydrazine is metered into the catalyst bed at a rate sufficient to provide the required output power. The speed of the turbine is controlled by varying the rate at which the liquid hydrazine flows into the gas generator, increasing the flow rate to increase the speed of the turbine, and decreasing the flow rate to decrease the speed of the turbine. It will be recognized that having precise control of the speed of the turbine, and the volume and other characteristics of the gas produced by the gas generator, is an important requirement.
The combustion process by which the hydrazine fuel is converted to warm gas has proven to be extremly destructive to the catalyst bed, drastically limiting the life of the gas generator. The hydrazine liquid which flows into the gas generator causes an oscillatory movement in the catalyst bed resulting in destructive vibration of the catalyst, which is in the form of particles having a particular size and shape. The vibration in the catalyst bed causes the particles to be broken down into less useful, smaller particles, and finally into dust. The vibration problem is particularly destructive immediately adjacent the spray nozzles through which the liquid hydrazine is introduced into the gas generator. As the catalyst particles in the bed break up, the dust produced migrates through the bed which becomes clogged rapidly by the dust, thus increasing the pressure drop across the gas generator. The vibration problem in the bed is so severe as to cause significant abrasion on the interior of the gas generator.
In order to minimize this problem, in the past biasing springs have been used to force the catalyst bed, typically enclosed in a porous cylindrical structure, against housing support structure to compensate for compacting, bed degradation and/or manufacturing tolerances, and to reduce the amount of vibration occurring therein due to the hydrazine spray after an initial use of the system. While this technique allows reuse of the gas generator without the necessity to completely rebuild the generator after each use, the number of uses the generator is capable of without requiring rebuilding and replacement of the catalyst is still far less than satisfactory. The initial degradation of the catalyst bed is somewhat slower when springs are used, but after initial wear in the catalyst bed occurs the catalyst degrades at a quickly increasing rate. It has been found that once flow-induced vibration is set up in the bed, the action of the spring in certain circumstances can further enhance bed vibration.
Another factor in the problem is the ambient temperature at which the gas generator is operating. While catalyst degradation is merely unsatisfactory at ambient temperatures of 70.degree. F., at lower operating temperatures the catalyst breakdown rate drastically increases to make the expected life time of the gas generator particularly short, rendering the device no longer just unsatisfactory but rather unacceptable. While the number of multiple starts possible at a low ambient operating temperature is a maximum of 5-10, it must be noted that the performance of the gas generator as an engine start device sharply diminishes resulting in a markedly slower turbine acceleration, increasing start time, and drastic catalyst weight loss through increasing oscillatory vibration.
Since one of the most important applications of the gas generator is as an emergency power source for a military aircraft, which typically operates at a high altitudes having a low ambient temperature, it can be seen that this type of gas generator will have a fairly short operating life after which a complete rebuilding and replacement of the catalyst bed is necessary. Since the gas generator has application as a component in the emergency power system for the aircraft, any failure in the gas generator could result in loss of the aircraft due to failure to restart the engine or operate the electric or hydraulic system of the plane. Furthermore, an operational test of the gas generator cannot be performed conveniently because the hydrazine fuel must be stored in sealed tanks to prevent leakage. Testing of the gas generator would cause this seal to be ruptured, requiring that the tank be replaced. Also, an operational test would expose ground support personnel to the toxic exhaust products of hydrazine combustion. It can therefore be appreciated that the gas generator would be required to be rebuilt after virtually every use to insure that loss of the aircraft does not result from decreased performance of the gas generator due to catalyst degradation from oscillatory vibration.
Although failure of the gas generator resulting in possible loss of an aircraft is of paramount importance, another major concern in premature catalyst degradation is the high cost of rebuilding the gas generator and replacing the catalyst. To rebuild the gas generator, it is necessary to remove the generator from the aircraft and completely disassemble it to replace the catalyst. Such an operation is labor intensive, requiring a substantial amount of time from a skilled technician. In addition, the Shell 405 or like catalyst used in the gas generator is extremely expensive. Although only a small amount of catalyst is contained in the bed, the cost of even the small amount of catalyst may be greater than the cost of the mechanically complete generator. Finally, an additional cost in military applications is that the aircraft must either have a large number of spare gas generators on hand, or else be unavailable for service while the gas generator is being rebuilt.
Thus, it can be seen that a strong need exists for a gas generator having an extended catalyst life allowing the generator to be reused a large number of times while requiring only minimal maintenance between uses. The gas generator must protect the catalyst particles from destructive oscillatory vibration. The gas generator must be capable of functioning reliably at low temperatures, and of providing multiple starts at these low temperatures. Finally, it is desirable that whatever solution is found be adaptable to retrofit existing gas generators to overcome the above-described problems.